Lithium ion secondary batteries or lithium secondary batteries can achieve high energy density, and therefore attract attention as power supplies for cellular phones and notebook computers and large-sized power supplies for power storage and power supplies for automobiles.
Lithium ion secondary batteries or lithium secondary batteries can achieve high energy density, but when they are large-sized, the energy density is enormous, and therefore, higher safety is required. For example, in applications for large-sized power supplies for power storage and power supplies for automobiles, particularly high safety is required. In these applications, safety is ensured by improving the structural design of cells, packages, and the like, disposing protection circuits, selecting electrode materials, additives with an overcharge-preventing function, and the like, strengthening the shutdown function of separators, and the like.
On the other hand, in lithium ion secondary batteries, usually, aprotic solvents, such as cyclic carbonates and chain carbonates, are used as electrolytic solution solvents. These carbonates have a high dielectric constant and high ionic conductivity of lithium ions, but have a low flash point and are flammable.
Therefore, a technique using, as an additive added to an electrolytic solution, a substance that reductively decomposes at a higher potential than carbonates used as electrolytic solution solvents and produces an SEI (Solid Electrolyte Interface), which is a protective film with high lithium ion permeability, is known. This SEI can improve charge and discharge efficiency, cycle characteristics, and safety. In addition, the irreversible capacity of carbon materials and oxide materials can be reduced by the SEI.
In addition, one example of a method for further increasing the safety of a lithium ion secondary battery includes making an electrolytic solution flame-retardant. Patent Literature 1 discloses an organic electrolytic solution secondary battery using a phosphate triester as the main solvent of an organic electrolytic solution, and using a carbon material for a negative electrode. Patent Literature 2 discloses that safety can be improved by using a phosphate triester as the organic solvent of an electrolytic solution.
Patent Literature 3 discloses a nonaqueous electrolyte secondary battery including a positive electrode capable of being charged and discharged, a nonaqueous electrolyte containing a lithium salt, and a negative electrode capable of being charged and discharged, wherein the above nonaqueous electrolyte contains at least one selected from the group consisting of a phosphate ester, a halogen-containing substituted phosphate ester, and a condensed phosphate ester. Patent Literature 4 discloses that an electrolytic solution with low viscosity and excellent low temperature characteristics is obtained by using a mixed solvent of a particular halogen-substituted phosphate ester compound and a particular ester compound as an electrolytic solution solvent. Patent Literature 5 discloses a method for manufacturing a nonaqueous electrolyte battery in which a nonaqueous electrolyte battery is manufactured using a nonaqueous electrolyte obtained by adding vinylene carbonate and 1,3-propane sultone. Patent Literature 6 discloses that the nonaqueous electrolyte of a nonaqueous electrolyte battery contains 5% by mass or more of phosphate esters having at least one fluorine atom in the molecular chain based on the total mass of the nonaqueous electrolyte, and has an electrolyte salt concentration of 1 mol/L or more and a viscosity at 20° C. of less than 6.4 mPa·s. It is disclosed that the nonaqueous electrolyte battery includes a nonaqueous electrolyte with flame retardancy or self-extinguishing properties, and has good charge and discharge characteristics.
Patent Literature 7 discloses an electrolytic solution for a nonaqueous battery, including a particular phosphate ester derivative, a nonaqueous solvent, and a solute. Patent Literature 8 discloses that an electrolytic solution that is excellent in conductivity and reduction resistance, and exhibits high flame retardancy even in a small amount of blending is obtained by using a fluorophosphate ester compound for a nonaqueous electrolytic solution for a battery.
Patent Literature 9 discloses that when an electrolytic solution contains a solvent containing halogenated ethylene carbonate and at least one phosphorus-containing compound selected from the group consisting of a phosphate ester, a phosphate ester, and a phosphazene compound, chemical stability can be improved also at high temperature. Patent Literature 10 discloses a nonaqueous electrolytic solution obtained by dissolving a lithium salt in a nonaqueous solvent containing a phosphate ester compound, a cyclic carbonate containing a halogen, and a chain carbonate. Patent Literature 11 discloses that a nonaqueous electrolytic solution containing an organic solvent containing 0.5 to 30% by volume of a fluorine-containing phosphate ester and an electrolyte salt is nonflammable and flame retardant, and therefore is useful as an electrolytic solution for a lithium secondary battery. In addition, it is disclosed that the solubility of the electrolyte salt is high, and when the nonaqueous electrolytic solution is used for a battery, the discharge capacity is large, and the charge and discharge cycle characteristics are excellent.